The present invention relates to adaptive grating lobe suppression. In particular, based on measurements of the object field, clutter from grating lobes is detected, and if present, reduced or removed.
Grating lobes are a significant source of clutter for laterally under-sampled arrays. The spacing between the array elements needs to be sufficiently small to avoid or at least limit the amount of lateral under sampling. But, if the number of beam former channels is limited, which is particularly the case for multi-dimensional arrays, smaller element spacing results in a smaller aperture. Smaller apertures provide less lateral resolution and signal-to-noise ratio. For example, a 64×64 two-dimensional array with half wavelength spacing, results in 4,096 elements. To maintain the half wavelength or even wavelength spacing, a large number of beam-forming channels are required. To reduce the number of beam-forming channels in alternative embodiments, the multi-dimensional array is sparsely sampled, such as providing coarsely spaced element transducers. For example, a 32×32 element array is provided with one wavelength spacing. However, sparse spacing of the elements results in increased grating lobes. Grating lobes result in tissue or other structure spaced away from a region of interest contributing to the signal at the region of interest (i.e., undesired clutter).
Various approaches have been suggested to increase the aperture size without sparse sampling, such as including beamformer electronics in the transducer to reduce the number of cables extending from the transducer to the ultrasound system. Time division multiplexing and sub-array beam forming are provided at the transducer. However, the sophisticated electronics in the transducer are expensive and may result in data degradation.
U.S. Pat. No. 5,549,111 discloses grating lobe reduction through variable frequency techniques. The imaging center frequency, such as the transmit and associated receive frequency, is reduced as a function of an increasing steering angle. This technique assumes reflectors within the grating lobe fields contribute clutter. However, no, few or low echogenic targets may exist in a grating lobe field so that a variable frequency is not needed.
Adaptive techniques have been provided for phase aberration correction. The delay and apodization profiles are altered as a function of the differences in speed of sound through different tissues along a main lobe transmission region. Using complex correlation processing, the location of a peak value is determined to identify the desired delay and apodization profiles. The transmit and receive scanning then accounts for differences in tissue structure as a function of scan line for the main lobe energy transmissions. This technique may not reduce grating lobes.